This invention relates to a pressure or differential pressure measuring device wherein a deformation of a flexible sensing disk, such as metallic diaphragm in response to a pressure is detected, and more particularly, to a new disk material and a device for supporting the flexible disk for use in the pressure or differential pressure measuring device.
Conventionally, a substantially rigid metal ring is used to fixedly secure the sensing diaphragm to the housing. The peripheral end of the diaphragm is connected to the metal ring by welding, however, in this case the initially flat diaphram may be corrugated or the tension exerted thereon may not be uniform due to thermal expansion caused by the welding. These drawbacks should be eliminated in order to permit desirable operation of the diaphragm.
In order to obviate these drawbacks, a conventional differential pressure measuring device, shown in FIG. 1, includes a differential pressure detecting portion 1 formed in a generally cylindrical frame, and also provided are a first housing 2 and a second housing 3, into which insulating materials 4 and 5 made of glass or ceramic are filled. Each inner axial surface of the insulating materials is in a hemi-spherical shape to which metal foils 6 and 7 are secured to function as capacitor plates. The first and the second housings 2 and 3 are symmetrically formed to confront the metal foils with each other. Contacting portions of these housings 2 and 3 are provided with annular ridges 9 and 91, and annular grooves 10 and 101 having wedge-like cross section. Inner contacting portions of these housings are provided with conical recesses 110 and 111 having a height d. A sensing diaphragm 8 is interposed between the housings 2 and 3 and is fixed thereto at the ridge portions 9 and 91 by welding. A first measuring chamber 11 is determined by a space defined between the spherical surface of the first insulation 4 and the measuring diaphragm 8, and a second measuring chamber 12 is determined by a space defined between the spherical surface of the second insulation 5 and the sensing diaphragm 8. The differential pressure detecting portion 1 is supported by first and second casings 13 and 14 both of which are tightly secured to each other by fixing bolts 15 and 15. First and second pressure chambers 18 and 19 are provided between the first casing and the first insulation and between the second casing and the second insulation, respectively. An input pressure is introduced into a first measuring chamber 11 through a hole 16 formed in the first casing 13, the first pressure chamber 18 and a bore 20 formed in the first insulation 4, and a second input pressure or reference pressure is introduced into a second measuring chamber 12 through a hole 17 formed in the second casing 14, the second pressure chamber 19 and a bore 21 formed in the second insulation 5.
When a pressing force P is applied to the first and the second housings 2 and 3 by fastening the first and second casings 13 and 14 together by means of the bolts 15, since the peripheral ends of the housings 9 and 91 are tapered, the peripheral ends are urged radially outwardly due to the pressing force P, to reduce the height d while increasing the diameter of the ridge portion so that tensile stress is applied to the sensing diaphragm radially outwardly to thereby eliminate non-uniform or local tension of the diaphragm.
With this structure, the supporting plates 15 and 16 and a plurality of bolts 17 around the periphery thereof are required, so that a compact structure is not obtainable. Further, the thermal expansion coefficient of the housings 2 and 3, made of stainless steel, is 17.times.10.sup.-6 to 18.times.10.sup.-6, whereas the thermal expansion coefficient of the supporting plates 15 and 16 and the bolts 17 made of steel is 10.times.10.sup.-6 to 11.times.10.sup.-6, so that the clamping force may be changed with changing temperature due to the relatively large difference of the thermal expansion coefficient, resulting in the degradation of the sensing characteristic of the diaphragm.
Another method of assembling the pressure measuring device has been proposed wherein the sensing diaphragm is welded to the outer peripheral surface of the housings 2 and 3 while maintaining a predetermined tension. However, such a method is disadvantageous in that the diaphragm may be deformed due to the welding, so that it would be rather difficult to obtain a sensing diaphragm to which stable and uniform tension is applied.